Controlling the rheology of a paint both in the stage of its manufacture, and during its transport, storage or use, remains a priority at the current time. The wide variety of constraints observed in each of these steps relates to a multiplicity of different rheological properties. Nevertheless, it is possible to summarise the requirement of the skilled man in the art in obtaining an effect of the thickening of the paint, both for reasons of stability over time, and for a possible application to a vertical surface, lack of spattering during use, or of sagging after application, etc. As a consequence, the products which contribute to this regulation of the rheological properties have been designated by the term “thickening agents”.
Historically, since the 1950s cellulose-based gums and products have been used, one of the essential characteristics of which is their high molecular weight.
However, these compounds have a number of disadvantages, such as their instability over time (see document U.S. Pat. No. 4,673,518), the need to use a large quantity of them (see document EP 0 250 943 A1), and their production costs, notably in terms of waste treatment (see document U.S. Pat. No. 4,384,096).
Thickening agents called “associative” thickening agents were then created: these are water-soluble polymers having insoluble hydrophobic groups. Such macromolecules have an associating character: when introduced into water, the hydrophobic groups tend to assemble in the form of micellar aggregates. These aggregates are linked together by the hydrophilic parts of the polymers: a three-dimensional network is then formed which causes the viscosity of the medium to be increased. The operating mechanism and their characteristics are now well known and described, for example in the documents “Rheology modifiers for water-borne paints” (Surface Coatings Australia, 1985, pp. 6-10) and “Rheological modifiers for water-based paints: the most flexible tools for your formulations” (Eurocoat 97, UATCM, vol. 1, pp 423-442).
Among these associative thickening agents, a technological platform is known which contains particular emulsions known as “HASE” (Hydrophobically modified Alkali-Soluble Emulsions). These contain polymers of (meth) acrylic acid, of an ester of these acids and of an associative monomer consisting of an oxyalkylated chain terminated by a hydrophobic group.
In the case of these associative monomers, the choice of hydrophobic group determines the varied rheological properties. The following patent applications filed by Coatex™ may be cited with this regard: EP 0 577 526 A1, which describes a fatty chain with linear or branched units of the alkyl and/or aryl type, having 26 to 30 carbon atoms, to develop high viscosities under a low shearing gradient, and EP 1 778 797 A1, which describes a branched terminal chain comprising 10 to 24 carbon atoms, to improve the pigmentary compatibility, and increase the viscosity generally.
However, associative thickening agents—and notably HASE—have a tendency to cause viscosities which may increase over the storage time, from the time when they are introduced into a paint. It is, indeed, well known that the thickening power which they develop tends to increase over time, when they are associated in the paint with binders which require no or little in the way of coalescence aid agents: this trend can generally be observed 8 days after formulation. Such variance is not desirable, since it is synonymous with a loss of control of the paint's rheological profile.